Not sure I really understand your problem, but here's some code to help
you out:
N24; %length of zero padded fft
x=0:100;
f=sin(2*x) + sin(3*x) + sin(4*x);
Ftshift(abs(fft(f,N)));
w=-pi:2*pi/N:pi-(pi/N); %x scaled from -pi to pi
plot(w,F)
I hope that helps, feel free to e-mail me privately if you want an explanation.